Processes for producing powders having primary particle sizes below 100 nm (nanosize powders) have attracted increasing interest in recent years since these powders have the potential to enable completely new materials, for example ceramics or composites, based on them to be developed. As in the case of the submicron powders (particle diameters of from 0.1 to 1 .mu.m) already available, high demands in terms of quality are also made of nanosize powders but these demands are different depending on material and application. For ceramic powders, important criteria are, for example:
high chemical purity PA1 phase stability PA1 powder density PA1 crystallinity PA1 particle size distribution and particle morphology PA1 specific surface area PA1 state of agglomeration. PA1 (a) a suspension containing amorphous or partially crystalline nanosize particles is produced in a conventional manner from precursors for the nanosize particles, where the nanosize particles are produced in a solvent which has no solvent capability, or only a low solvent capability, for the particles and in the presence of at least one surface-blocking substance, or PA1 (b) an already formed powder comprising amorphous or partially crystalline nanosize particles is suspended in the solvent specified under (a) in the presence of the surface-blocking substance or substances specified under (a), or PA1 (c) a sol containing amorphous or partially crystalline nanosize particles is suspended in the solvent specified under (a) in the presence of the surface-blocking substance or substances specified under (a); and the suspension thus produced is subjected to conditions which lead to a densification and/or crystallization of the nanosize particles.
The last point in particular is of great importance for the use of nanosize powders in powder metallurgical processing and manufacturing processes. In general, the powder particles should have a density which is as high as possible and/or possess crystalline structures. The agglomerates which are inevitably present should have a nature such that they can be broken up again to their primary particle size during processing. The potential of nanosize powders can be optimally utilized only subject to these prerequisites. This means that soft agglomerates are required. Particles which allow the state of agglomeration between nanosize particles to be adjusted are therefore necessary. This can be carried out during the synthesis or in a downstream process.
Physical and chemical processes for producing nanosize (ceramic) powders are described in the literature. The physical processes are divided into three categories, namely vacuum, gas-phase and condensed-phase syntheses. However, their applicability is restricted by the low material conversion to the production of small amounts of powder.
Processes which include chemical reactions are becoming increasingly important in the powder synthesis, for example hydrothermal synthesis, precipitation reactions, flame hydrolysis, plasma synthesis, the sol-gel process or emulsion processes.
In the hydrothermal synthesis, inorganic salts are converted by means of precipitation reactions under increased pressure and elevated temperatures (above the critical data of the solvent) into the corresponding oxide, hydrated oxide or hydroxide. Setting the optimum reaction parameters (pH, type and concentration of the starting compounds, pressure, temperature) enables crystallite sizes of about 20 nm to be achieved. However, a disadvantage in this process is the formation of agglomerates which can no longer be broken up. These agglomerates are formed as a result of metal-OH groups present on the particle surface undergoing condensation reactions during drying and calcination of the powder. Since agglomerate formation is generally not reversible the potential of this technique can at present be utilized only to a restricted extent.
Flame hydrolysis is a standard method of producing aerosils. It gives high powder yields and can be applied to many materials. In this process, volatile compounds such as SiCl.sub.4, TiCl.sub.4 or ZrCl.sub.4 are reacted in a hydrogen/oxygen flame to give very fine oxide particles. Although oxide powders having particle sizes of from 5 to 50 nm can be produced by means of flame hydrolysis, a disadvantage of this process is the great degree of agglomerate formation, since the cohesion between the particles increases greatly with decreasing particle size. Redispersion of these powders to their primary particle size is usually possible to only a small extent, if at all.
Plasma synthesis enables not only oxidic powders but also nitrides and carbides to be produced. In this process, for example, metal powders or suitable metal compounds are vaporized in an inductively coupled plasma and reacted with ammonia to produce nitrides or with methane to produce carbides. This process enables highly pure, very fine spherical powders to be produced and, when the reaction parameters are optimally set, it gives particles having diameters of about 5 nm which, although they are agglomerated, have only few solid bridges between the particles. However, this is a technically very complicated process which is associated with a high outlay in terms of apparatus.
A further process is the sol-gel process. In this method, suitable starting compounds such as reactive metal alkoxides are hydrolyzed and condensed in a solvent to form a sol (soluble oligomers or polymers or colloidal suspension). Further reaction forms a gel (solid) which can be converted by thermal after-treatment into a crystalline powder. Adjustment of particular parameters makes it possible to control the reaction in such a way that sols having particle sizes far below 50 nm can be produced. A disadvantage of this process is the formation of amorphous materials which have to be converted into a crystalline product by means of a thermal after-treatment. Owing to the high density of OH groups on the particle surface, condensation reactions result in neck formation between the particles (aggregation=hard agglomerates) which makes it impossible to break the powders down to the primary particle size during processing.
A relatively new route to the precipitation of very fine ceramic powders is the emulsion technique. In this method, an aqueous phase in the form of very fine droplets is dispersed in a liquid which is not miscible with water. Both the droplet size and the stability of the emulsions are dependent on many factors which, however, have only begun to be studied for powder syntheses.
A prerequisite for the formation of powders using the emulsion technique is that the dispersed aqueous phase can be converted into a solid phase by suitable chemical reactions such as precipitation or condensation reactions. An important role in the formation of an emulsion is played by surface-active substances (emulsifiers) with whose help a salt solution can be emulsified to form very fine droplets in a hydrocarbon. The water droplets emulsified in this way can be regarded as submicroscopic minireactors which have the same properties as macroscopic solutions. The metal hydroxides or oxides can be precipitated by increasing the pH. This is achieved, for example, by passing ammonia gas into the emulsion or by the addition of organic bases which have to be soluble in the dispersion medium. To convert the liquid phase into a solid phase, the water is removed by azeotropic distillation. The particles formed are thereby densified. Agglomeration of the particles is largely prevented by the presence of the emulsifiers which shield the reactive surfaces.
Although high-quality nanosize powders can be produced via emulsions, the volume yield is frequently low in such emulsion processes, so that these processes are not exploited industrially for powder production. A decisive reason for the low volume yield in emulsion processes is that emulsions always involve two-phase systems but the precursors are introduced only via one phase.
As shown above, there are many synthetic variants for very fine powders. However, an unsolved problem is control of agglomeration, particularly in the densification and/or crystallization of nanosize particles (diameter 1-100 nm).